This invention relates to driver circuits for high-speed electromagnets, and more particularly to driver circuits for energizing electromagnets of the type which actuate hammers or needles in high-speed impact printers employed in data processing systems.
Driver circuits for such high-speed electromagnets have been required to satisfy severe requirements, including providing fast magnetization of the electromagnet and providing a high level of repeatability in operation so as to have minimum variation in the print quality and actuation time during successive energizations of the electromagnet.
One form prior art of driver circuit for satisfying such requirements utilizes a voltage controlled energization system providing a first high energization voltage and a second lower holding voltage, applied during appropriate and successive periods. Such system must have precise voltage controlled power supplies. In addition, to assure, during each energization, the complete magnetization of the magnetic circuit, a substantial amount of energy must be wasted, with accompanying oversized and excessively costly power supplies.
Another form of prior art driver circuit for satisfying such requirements utilizes an unregulated energization system in which the energization is held until a preestablished current level is reached, whereupon the energization is switched off as the electromagnet winding is shorted, at least for a predetermined duration, so that the electromagnet continues to be magnetized. This form of driver circuit is particularly effective if the energization time necessary for saturating the core of the electromagnet is less than the time required for the electromagnet armature to move. However, a very high supply voltage is required, with the accompanying requirement for employing components that must withstand such high voltages.
Accordingly, it is the principal object of the present invention to provide a simple driver circuit which satisfies the above specified requirements; namely, providing fast magnetization of the associated electromagnet and providing outstanding repeatability in operation with limited variation in the print quality and actuation time of the electromagnet during successive energizations.
Another object of the present invention is to provide a driver circuit for an electromagnet which operates with supply voltages of moderate level.
Another object of the present invention is to provide a driver circuit for an electromagnet which operates with supply voltages that are not stabilized.
Another object of the present invention is to provide a circuit for supplying the magnetomotive force necessary to saturate the magnetic circuit of an electromagnet, while assuring under conditions of variation in the level of the supply voltage the necessary attractive force and momentum required by the armature of such electromagnet.
Another object of the present invention is to provide a driver circuit for an electromagnet in which the amount of resistive losses in the winding is substantially constant during each energization cycle thereby providing for using windings of optimum size for dissipating the consequent thermal energy to the surroundings.